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  • 1
    Language: English
    In: Marine Biology, 2012, Vol.159(11), pp.2573-2581
    Description: Analysing long-term diatom data from the German Bight and observational climate data for the period 1962–2005, we found a close connection of the inter-annual variation of the timing of the spring bloom with the boreal winter atmospheric circulation. We examined the fact that high diatom counts of the spring bloom tended to occur later when the atmospheric circulation was characterized by winter blocking over Scandinavia. The associated pattern in the sea level pressure showed a pressure dipole with two centres located over the Azores and Norway and was tilted compared to the North Atlantic Oscillation. The bloom was earlier when the cyclonic circulation over Scandinavia allowed an increased inflow of Atlantic water into the North Sea which is associated with clearer, more marine water, and warmer conditions. The bloom was later when a more continental atmospheric flow from the east was detected. At Helgoland Roads, it seems that under turbid water conditions (= low light) zooplankton grazing can affect the timing of the phytoplankton bloom negatively. Warmer water temperatures will facilitate this. Under clear water conditions, light will be the main governing factor with regard to the timing of the spring bloom. These different water conditions are shown here to be mainly related to large-scale weather patterns. We found that the mean diatom bloom could be predicted from the sea level pressure one to three months in advance. Using historical pressure data, we derived a proxy for the timing of the spring bloom over the last centuries, showing an increased number of late (proxy-) blooms during the eighteenth century when the climate was considerably colder than today. We argue that these variations are important for the interpretation of inter-annual to centennial variations of biological processes. This is of particular interest when considering future scenarios, as well to considerations on past and future effects on the primary production and food webs.
    Keywords: Algal Blooms ; Atmospheric Forcing ; Climate Change ; Ocean-Atmosphere System ; Phytoplankton ; Primary Production ; Sea Level Pressure ; Plankton ; Winter ; Weather ; Data Processing ; Oscillations ; Grazing ; Climate ; Zooplankton ; Phytoplankton ; Diatoms ; Atmospheric Circulation ; Water Temperature ; Primary Production ; Light Effects ; Pressure ; Food Webs ; Bacillariophyceae ; Ane, Atlantic, Azores ; Ane, Norway ; Ane, North Sea ; Ane, Germany, German Bight ; Ane, Scandinavia ; Ane, Germany, Schleswig-Holstein, North Frisian Is., Helgoland ; An, North Atlantic, North Atlantic Oscillation ; Marine ; Habitat Community Studies ; Ecology/Community Studies ; Ecology;
    ISSN: 0025-3162
    E-ISSN: 1432-1793
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  • 2
    In: Nature, 2011, Vol.471(7336), p.91
    Description: The Milankovitch theory states that global climate variability on orbital timescales from tens to hundreds of thousands of years is dominated by the summer insolation at high northern latitudes (1,2). The supporting evidence includes reconstructed air temperatures in Antarctica that are nearly in phase with boreal summer insolation and out of phase with local summer insolation (3-5). Antarctic climate is therefore thought to be driven by northern summer insolation (5). A clear mechanism that links the two hemispheres on orbital timescales is, however, missing. We propose that key Antarctic temperature records derived from ice cores are biased towards austral winter because of a seasonal cycle in snow accumulation. Using present-day estimates of this bias in the 'recorder' system, here we show that the local insolation can explain the orbital component of the temperature record without having to invoke a link to the Northern Hemisphere. Therefore, the Antarctic ice-core-derived temperature record, one of the best-dated records of the late Pleistocene temperature evolution, cannot be used to support or contradict the Milankovitch hypothesis that global climate changes are driven by Northern Hemisphere summer insolation variations.
    Keywords: Solar Radiation -- Measurement ; Temperature Measurement -- Comparative Analysis ; Temperature Measurement -- Methods;
    ISSN: 0028-0836
    E-ISSN: 14764687
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  • 3
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, March 1, 2013, Vol.373, p.152(11)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.palaeo.2012.08.006 Byline: Gerrit Lohmann, Bernd R. Schone Keywords: Climate proxy; North Atlantic climate variability; Teleconnections; Proxy data; Atmospheric circulation Abstract: Pronounced decadal climate oscillations are detected in a multi-centennial record based on shell growth rates of the marine bivalve mollusk, Arctica islandica, from Iceland. The corresponding analysis of patterns in sea level pressure and temperature exhibit large-scale teleconnections with North Atlantic climate quantities. We find that the record projects onto blocking situations in the northern North Atlantic. The associated circulation shows a low-pressure signature over Greenland and the Labrador Sea and a high-pressure system over Western Europe associated with northeasterly flow towards Iceland and weakening in the westerly zonal flow over Europe. It can be speculated that such circulation affects food availability controlling shell growth. On multidecadal time scales, the record shows a pronounced variability linked to North Atlantic temperature. In our record, we find enhanced variability of the shell growth rates on multidecadal time scales, and it appears that this oscillation has high amplitudes in the 16th to 18th century also consistent with marine alkenone data. It is conceivable that these climate oscillations, also linked to sea ice export and enhanced blocking, are a more pronounced feature during times when the climate was relatively cold. Article History: Received 1 November 2011; Revised 4 August 2012; Accepted 12 August 2012
    Keywords: Atmospheric Circulation -- Analysis ; Climate Cycles -- Analysis ; Climate -- Analysis
    ISSN: 0031-0182
    Source: Cengage Learning, Inc.
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  • 4
    In: Nature, 2014
    ISSN: 0028-0836
    E-ISSN: 1476-4687
    Source: Nature Publishing Group
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  • 5
    Language: English
    In: Nature, 21 August 2014, Vol.512(7514), pp.290-4
    Description: During glacial periods of the Late Pleistocene, an abundance of proxy data demonstrates the existence of large and repeated millennial-scale warming episodes, known as Dansgaard-Oeschger (DO) events. This ubiquitous feature of rapid glacial climate change can be extended back as far as 800,000 years before present (BP) in the ice core record, and has drawn broad attention within the science and policy-making communities alike. Many studies have been dedicated to investigating the underlying causes of these changes, but no coherent mechanism has yet been identified. Here we show, by using a comprehensive fully coupled model, that gradual changes in the height of the Northern Hemisphere ice sheets (NHISs) can alter the coupled atmosphere-ocean system and cause rapid glacial climate shifts closely resembling DO events. The simulated global climate responses--including abrupt warming in the North Atlantic, a northward shift of the tropical rainbelts, and Southern Hemisphere cooling related to the bipolar seesaw--are generally consistent with empirical evidence. As a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets, minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere-ocean-sea-ice feedback in the North Atlantic. Our results, although based on a single model, thus provide a coherent concept for understanding the recorded millennial-scale variability and abrupt climate changes in the coupled atmosphere-ocean system, as well as their linkages to the volume of the intermediate ice sheets during glacials.
    Keywords: Ocean Circulation ; Climate Change ; Atmosphere ; Hypotheses ; Cold ; Heat ; Icebergs;
    ISSN: 00280836
    E-ISSN: 1476-4687
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  • 6
    In: Nature, 2003, Vol.424(6948), p.532
    Description: During the two most recent deglaciations, the Southern Hemisphere warmed before Greenland. At the same time, the northern Atlantic Ocean was exposed to meltwater discharge, which is generally assumed to reduce the formation of North Atlantic Deep Water. Yet during deglaciation, the Atlantic thermohaline circulation became more vigorous, in the transition from a weak glacial to a strong interglacial mode. Here we use a three-dimensional ocean circulation model to investigate the impact of Southern Ocean warming and the associated sea-ice retreat on the Atlantic thermohaline circulation. We find that a gradual warming in the Southern Ocean during deglaciation induces an abrupt resumption of the interglacial mode of the thermohaline circulation, triggered by increased mass transport into the Atlantic Ocean via the warm (Indian Ocean) and cold (Pacific Ocean) water route. This effect prevails over the influence of meltwater discharge, which would oppose a strengthening of the thermohaline circulation. A Southern Ocean trigger for the transition into an interglacial mode of circulation provides a consistent picture of Southern and Northern hemispheric climate change at times of deglaciation, in agreement with the available proxy records.
    Keywords: Sea Ice ; Melt Water ; Palaeoceanography ; Ice Melting ; Deglaciation ; Thermohaline Circulation ; Ocean Circulation ; Paleoclimatology ; Deglaciation ; Thermohaline Circulation ; Ocean Circulation Models ; World Ocean ; PS, Antarctic Ocean ; An, Atlantic ; A, Atlantic ; Marine ; Structure/Dynamics/Circulation (551.465) ; Palaeo-Studies ; Physical Oceanography ; Prehistoric and the Quaternary Geological Period (551.583.3) ; Models. Model Experiments (551.460.072);
    ISSN: 0028-0836
    E-ISSN: 1476-4687
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  • 7
    Language: English
    In: Proceedings of the National Academy of Sciences of the United States of America, 22 May 2018, Vol.115(21), pp.5365-5370
    Description: The Pacific hosts the largest oxygen minimum zones (OMZs) in the world ocean, which are thought to intensify and expand under future climate change, with significant consequences for marine ecosystems, biogeochemical cycles, and fisheries. At present, no deep ventilation occurs in the North Pacific due to a persistent halocline, but relatively better-oxygenated subsurface North Pacific Intermediate Water (NPIW) mitigates OMZ development in lower latitudes. Over the past decades, instrumental data show decreasing oxygenation in NPIW; however, long-term variations in middepth ventilation are potentially large, obscuring anthropogenic influences against millennial-scale natural background shifts. Here, we use paleoceanographic proxy evidence from the Okhotsk Sea, the foremost North Pacific ventilation region, to show that its modern oxygenated pattern is a relatively recent feature, with little to no ventilation before six thousand years ago, constituting an apparent Early-Middle Holocene (EMH) threshold or "tipping point." Complementary paleomodeling results likewise indicate a warmer, saltier EMH NPIW, different from its modern conditions. During the EMH, the Okhotsk Sea switched from a modern oxygenation source to a sink, through a combination of sea ice loss, higher water temperatures, and remineralization rates, inhibiting ventilation. We estimate a strongly decreased EMH NPIW oxygenation of ∼30 to 50%, and increased middepth Pacific nutrient concentrations and carbon storage. Our results () imply that under past or future warmer-than-present conditions, oceanic biogeochemical feedback mechanisms may change or even switch direction, and () provide constraints on the high-latitude North Pacific's influence on mesopelagic ventilation dynamics, with consequences for large oceanic regions.
    Keywords: Holocene ; North Pacific ; Intermediate Water ; Oxygen Minimum Zone ; Stable Isotopes
    ISSN: 00278424
    E-ISSN: 1091-6490
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  • 8
    Language: English
    In: Theoretical and Applied Climatology, 2017, Vol.128(3), pp.709-718
    Description: Large-scale atmospheric patterns are examined on orbital timescales using a climate model which explicitly resolves the atmosphere–ocean–sea ice dynamics. It is shown that, in contrast to boreal summer where the climate mainly follows the local radiative forcing, the boreal winter climate is strongly determined by modulation of circulation modes linked to the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) and the Northern/Southern Annular Modes. We find that during a positive phase of the AO/NAO the convection in the tropical Pacific is below normal. The related atmospheric circulation provides an atmospheric bridge for the precessional forcing inducing a non-uniform temperature anomalies with large amplitudes over the continents. We argue that this is important for mechanisms responsible for multi-millennial climate variability and glacial inception.
    Keywords: Storm Track ; Southern Annular Mode ; Marine Isotope Stage ; Boreal Winter ; Summer Insolation
    ISSN: 0177-798X
    E-ISSN: 1434-4483
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  • 9
    Language: English
    In: Palaeogeography, Palaeoclimatology, Palaeoecology, July 1, 2012, Vol.339-341, p.66(8)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.palaeo.2012.04.023 Byline: Luisa Cristini, Klaus Grosfeld, Martin Butzin, Gerrit Lohmann Keywords: Antarctica; Cenozoic; Numerical models; Drake Passage Abstract: We evaluate the opening of the Drake Passage (DP), between Antarctica and South America, and associated changes in ocean circulation as forcing factor for the onset of Antarctic glaciation near the Eocene-Oligocene transition (~34 million years ago). In this paper this hypothesis is tested through sensitivity experiments, using numerical models for the global ocean and atmosphere and for the Antarctic Ice Sheet. The response of the Antarctic continent to the opening of the DP and to the establishment of the Antarctic Circumpolar Current is examined. Two different climate states are reproduced with ocean gateway configurations similar to the Late Eocene and to the Late Oligocene, before and after the opening of the DP. A reduced southward heat flux and a decrease of surface temperature are found in the Antarctic realm when the DP is open. A more massive ice sheet develops on the continent in case of DP open compared to the configuration with closed DP. Article History: Received 29 July 2011; Revised 13 April 2012; Accepted 23 April 2012
    Keywords: Ocean Circulation -- Analysis ; Ocean Circulation -- Models
    ISSN: 0031-0182
    Source: Cengage Learning, Inc.
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  • 10
    In: Geophysical Research Letters, 28 August 2015, Vol.42(16), pp.6811-6818
    Description: Climate change can influence sea surface conditions and the melting rates of ice sheets; resulting in decreased deep water formation rates and ultimately affecting the strength of the Atlantic Meridional Overturning Circulation (AMOC). As such, a detailed study of the interactive role of dynamic ice sheets on the AMOC and therefore on global climate is required. We utilize a climate model in combination with a dynamic ice sheet model to investigate changes to the AMOC and North Atlantic climate in response to Intergovernmental Panel on Climate Change scenarios for RCP4.5 and RCP6. It is demonstrated that the inclusion of an ice sheet component results in a drastic freshening of the North Atlantic by up to 2 practical salinity units, enhancing high‐latitude haloclines and weakening the AMOC by up to 2 sverdrup (10 m/s). Incorporating a bidirectionally coupled dynamic ice sheet results in relatively reduced warming over Europe due to the associated decrease in heat transport. Ice sheet/climate feedbacks can occur on a decadal scale in the future Greenland melting causes the deep ocean circulation to decrease significantly The discrepancy caused by including an ice sheet causes cooling of up to 30%
    Keywords: Climate Modeling ; Amoc ; Ice Sheet Melting
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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